Polyphase slave motor



Nov. 18, 1958 A. N. BLANKENSHIP 2,861,236

POLYPHASE SLAVE MOTOR Filed March 19, 1956 'mvrox AL LEN N. NKENSHIP avgATTORNEYS United States Patent Ofifigg Patented Nov. 18, 1958 2,861,236POLYPHASE SLAVE Moron Allen N. Blankenship, Seattle, Wash. ApplicationMarch 19, 1956, Serial No. 572,426

3 Claims. or. 318-361) This invention relates to polyphase slave motorswhich are similar to the slave motor disclosed in my pending applicationSerial No. 277,858, filed March 21, 1952, now Patent No. 2,739,278,dated March 20, 1956, in that the stator winding is commuted and thecommutating brushes are mounted for piloted rotation independently ofboth the rotor and the commutator, the speed of the brushes being usedto control the speed of the motor.

The invention particularly aims to provide a polyphase variable speedmotor with a stepless wide range of speed and a; simplified means ofcontrol. As further objects the invention aims to provide such a motorwhich is di rectly reversible and has a high starting torque.

With yet additional objects and advantages in view which, with theforegoing, will appear and be understood in the course of the followingdescription and claims, the invention consists in the novel constructionand in the adaptation and combination of parts hereinafter described andclaimed.

Inthe accompanying drawings:

Figure 1 is a schematic of a means for varying the speed of a magneticfield.

Figs. 2 and 3 are schematics of two different polyphase motorembodiments of my invention, part of the rotor beingbroken away ineach'instance to show the respective'stub shaft for the brush carrier.

Fig; 4 is a fragmentary schematic of a desirable commutation arrangementfor my motors.

In Fig. 1 I have illustrated schematically a three-phase winding 10 of adelta type commuted to a commutator 11 having its bars 12 electricallyinsulated from one another. A brush carrier 13 is journaled on a shaft14 and carries three equally spaced brushes 15 cooperating with thecommutator. These brushes are supplied with threephase power as via sliprings 16. A pilot motor may be used to drive and control the brushcarrier.

It may be considered that the starting points of the three phases of thewinding 10 are at whatever three commutating bars are contacted by thebrushes. When the brushes 15 are at rest there is, of course, a rotatingmagnetic field whose speed is directly proportional to the frequency ofthe voltage supplied to the brushes. If the brush carrier is thenrotatedat a given speed the starting points of the three phases may beconsidered as moving along in unison with the three brushes, andaccordingly it is found that the field is moving at a new speeddetermined in R. P. M. by the formula, 120 f/p+b, wherein f is thefrequency of the supplied voltage in cycles/see, p is the number ofpoles, and b is the brush speed in R. P. M. If the generatedelectromotive forces and the brush speed are in the same direction thefield speed will be greater than when the brushes are at rest; likewise,if generated electromotive forces and the brush speed are in oppositedirections, the field speed will be less than when the brushes are atrest. This means of varying the magnetic field speed can be readilyapplied to polyphase induction motors.

Heretofore, polyphase induction motors have consisted supply and a rotorwhich in the majority of cases is of the so-called squirrel-cage type,although wound rotors are also sometimes used. In any regard, it hasbeen considered that the only possible methods of speed control 'forsuch a motor are by control of supply frequency,

rotor slip, or number of poles, or by control of a combination of thesethree factors; In Fig. 2 I have illustrated schematically a three-phaseinduction motor having its rotor'and stator denoted 18, 19. The statoris provided with the winding 10 and commutator 11 and the brush carrier13 is journaled for rotation independently of the rotor and commutator.This may be accomplished by journal-mounting the carrier on the shaft 20of the rotor, but best results are achieved by providing a stationarystub shaft 21 carried by the stator. This stub shaft corresponds to theshaft 14 in Fig. 1 and carries the slip rings 16.

In a conventional polyphase induction motor the frequency of the rotorcurrent is dependent upon the rate at which the stator flux is cut bythe rotor conductors. When the rotor is at a standstill the relativemovement between the stator flux and the rotor conductors is equal tothe speed of the stator flux, and accordingly, in such an instance thefrequency of the rotor current is the same as that of the supplyfrequency to the stator. However, in my Fig. 2 embodiment, when therotor is at a standstill and the brushes are rotating, the frequency ofthe rotor current is either greater or less than the supply frequencydepending upon the direction of brush rotation because the speed of thestator flux is varied by the speed of the brushes. Thus it is seen thatif the rotor in Fig. 2 is locked at rest and tapped a frequencyconverter results which is controlled by varying the speed of thebrushes.

Assuming now that the rotor in Fig. 2 is free to turn,

if the brushes are at rest the rotor will build up speed as in anyinduction motor until the electromotive force generated in the rotorcircuit is the right amount to supply sufiicient current to produce apropelling torque on the rotor and the mechanical losses combined.Assuming a constant load for the moment, the speed of the rotor will bedetermined by the speed of the stator flux, and since this flux speed isvariable by changing the brush speed, it is seen that the rotor speedcan be controlled by varying the brush speed. Change in rotor slip dueto load changes will modify this speed relation the same as inconventional induction motors. Thus it is seen that rotating the brushesin effect changes the so-called synchronous speed of the motor.

It will be apparent from the above discussion that if the rotor 15 inFig. 2 is a synchronous motor type excited with direct current via sliprings on the rotor shaft instead of being an induction-type rotor thatthe motor becomes a variable speed synchronous motor.

In the Fig. 2 embodiment the stator was the inducing component of themotor. However, a polyphase motor of very desirable characteristics canbe constructed with the rotor as the inducing member by using myrotating brush concept. A three-phase version of such a motor isillustrated schematically in Fig. 3 with the rotor and stator windings,for purposes of example, being given a delta delta relationship. Theshaft 22 for the rotor, denoted 23, is provided with the slip rings 24,for carrying power to the three phases 25 of the rotor winding and thestator 26, as before, is provided with the winding 10 and commutator 11.Also, as in the Fig. 2 embodiment, the brush carrier 13 is journaled forindependent rotation on a stub shaft 27 carried by the stator. Thebrushes 15 are electrically connected to center-taps of the three rotorphases 25 as by elongated flexible leads 28 or via slip rings mounted onthe stub shaft 27.

Reference to transformer principles is helpful in arriving at anunderstanding of the operation ofthemotor of Fig. 3, and at this time itshould be understood that the rotor and stator windings are purposelygiven a-one to one ratio and may be either simplex or duplexseries D. C.windings. Theserotor and stator windings may be considered for purposesofexplanation as'the primary and secondary windings, respectively;ofthree single-phase transformers connected delta delta.- Insuch aninstance the voltage induced in-the secondary phases'is equal to that ofthe primary phases and the voltage at the centers of the latter areatthe same potential as the'centers of the secondary phases so that ifthese phase centers were cross-tapped there would-be no current fiowbetween the primary and secondary. If the secondary phase tapswereshifted away from their phase centers, leads from these shiftedsecondary tap positions to the center-taps of the primary would nolonger interconnect equal potential points and a currentflow wouldresult therein from the higher to the lower potential. Maximum currentwould be achieved when the secondary phase taps were shifted 180electrical degrees either way with respect to the center-taps of theprimary phases.

In the Fig. 3 motor the rotatable brushes 15 cooperating with thecommutator bars 12 give a means to advance the tap positions of thesecondary or stator phases with respect to the center-taps'of theprimary or rotor phases. When the brushes are advanced to a newpositionby turning the brush carrier relative to the rotor, current re-'sults in the windings which produces rotor and stator magnetic fields.These are displaced with respect to one another and so in turn create atorque which urges the rotor to turn in the same direction as thebrushes were advanced. When the rotor reaches the new brush positionthere would no longer be a voltage difference at opposite ends of leads28, and hence current and torque would return to zero. If the brushes,instead of being advancedto a new position and stopped, are continuouslyrotated as by a pilot motor, voltage will be continuously induced in thestator winding and a constant advance of the brushes with respect to thecenter-taps of the rotor phases will result in a correspondingcontinuous current fio-w producing displaced rotating rotor and statorfields. These fields, as in the case of the field of winding in Fig. 1,will be rotating at greater or less speed than would be caused by supplyfrequency alone depending upon which direction the brushes are turningwith respect to the magnetic lines of force. At any rate the rotor fieldwill always lag the stator field a sufficient amount to create therequired torque, or in other words, for a given load, at a given speed,the brushes will lead the rotor at the correct angle to satisfy the loadrequirement. Motor speedis solely determined by the speed at which thebrush carrier is driven. Hence, in this regard the motor of Fig. 3differs from that of Fig. 2 since in the latter case motor speed wasdependent upon supply frequency as well as brush speed.

It'is possible that the rotor in the Fig. 3 embodiment could be soloaded that the brushes might turn through several revolutions relativeto the rotor, or in other words, the brushes might overrun the maximumtorque range. To prevent such an occurrence a stop pin 30 is mounted onthe rotor shaft and two limit pins 31 are mounted on the brush carrier.

To discourage arcing it is desirable to substitute a pair of brushes 15'for each brush 15 and to connect the pair to the ends of a respectiveinductance coil 32 (Fig. 4). These coils 32 are center-tapped to thebrush leads. The brushes of each pair are insulated apart and have awidth suchthat at least one of them will be in contact with a commutatorbar.

A small universal motor controlled through a Variack or rheostat servesas a satisfactory pilot motor 17 inmost applications. Where numerous ofmy slave motors need be synchronously controlled through wide ranges ofspeed, their respective bush systems could be locked together withselsyn receivers and controlled from one system transmitter.

It is thought that the invention will have been clearly understood fromthe, foregoing detailed description.

ductively related to the winding of said inducing member havingaone-to-one turn-ratiowith respect thereto, a brush carrierhavingequally spaced brushassembliescooperating with said commuted winding andelectrically connected together via said'winding of the-inducing member,saidbrush carrierbeing rotatable with respect tosaid inducing andinduced members, and stop means for limiting the rotation of saidcarrier with respect tosaidinducing member.

2. In a polyphase alternating'current motor, a rotor wound with. apolyphase primary winding to have an even number of' poles, a-statorhaving a polyphase secondary'windinginductively related to said primarywinding and having a one-t-o-one turnratio with respect thereto, acommutator tapped tosaid secondary winding, 2. bush carrier havingbrushes electrically connected to equal potential points of. the phasesof said primary winding and cooperating with said commutator, saidcarrier being independently rotatable with respect to said rotor andsaidcommutator, and stop means for limiting the: rotation. of said carrierwith respect to said rotor.

3. In a polyphase alternating current motor, a rotatable inducing memberprovided with a polyphase winding electrically connectedwith a polyphasealternating current source to create poles on said inducing member, aninduced member. havingacommuted polyphase winding inductively related tothe winding of said inducing member and having a one-to-one turn ratiowith respect thereto, a brush carrier having a respective brush assemblyfor each phase and cooperating with said commuted winding,.said.brushassemblies being electrically interconnected with said current sourceand said brush carrier being rotatable with respectto said inducing andinduced members, and stopmeans for limiting the rotation of said carrierwith respect to said inducing member.

References Cited in the file of this patent UNITED STATES PATENTS510,534 Gorges Dec. 12, 1893 748,907 Ziegenberg Jan. 5, 1904 1,254,902Hale Jan. 29, 1918 1,526,613 Stephenson Feb. 17, 1925 2,008,360 LellJuly 16, 1935 2,152,327 Rauhut Mar. 28, 1939 2,739,278 Blankenship Mar.20, 1956 FOREIGN PATENTS 498,968 Belgium Nov. 14, 1950'

